Well logging during drilling



Feb. l0, 1970 w. A. BOATMAN, JR

WELL LOGGING DURING DRILLING Filed Jan. l0, 1967 Il Il United States Patent O U.S. Cl. 73-153 18 Claims ABSTRACT OF THE DISCLOSURE A method of well logging during drilling is disclosed which involves the use of a characteristic, such as bulk density, of shale cuttings obtained from the drilling uid during drilling as an indication of sub-surface conditions. Apparatus for determining bulk density of shale cuttings is disclosed.

BACKGROUND During the drilling of oil wells a drilling uid is ordinarily employed to effect a hydrostatic pressure in the well bore suflicient to prevent gas and other fluids from blowing out of the well bore. The drilling uid is commonly known as drilling mud and is made to have sufficient density to create the hydrostatic pressure required, Mud weights are ordinarily varied during drilling depending on the pressures likely to be encountered.

One needs to predict in advance the pressures likely to be encountered in order to increase the mud weight appropriately, as well as to set casing down to appropriate depths to permit raising the density o f the mud suiciently without concurrently fracturing the sides of the well bore by hydrostatic pressure.

In the past, sub-surface conditions have been predicted by stopping drilling operations and then running instruments in the hole which analyze the strata through which the bit has penetrated. Another technique has involved drilling a core of the stratum being drilled and mechanically bringing the core to the surface for analysis, Thesetechniques involve several disadvantages. For example, the drilling operation must be suspended while the analysis is being performed; the techniques do not keep a running log during drilling; and trips in and out of the hole are required.

INVENTION SUMMARY The invention provides a well logging technique which admits of a running log while the hole is being drilled through shale. In one embodiment the well logging technique comprises successively obtaining samples of shale cuttings from the drilling fluid during drilling such that the samples are representative of the shale being drilled at successive depths; determining at least one characteristic, such as bulk density, of shale cuttings within each of the samples; and employing the characteristic of the shale cuttings as an indication of sub-surface conditions, particularly pressures likely to be encountered during drilling.

The invention also provides apparatus for measuring bulk volume and bulk density of shale cuttings obtained from the drilling uid during drilling operations in a rapid, convenient, and accurate manner.

DRAWING DESCRIPTION In the drawing, one embodiment of apparatus in accordance with the invention is shown in a schematic, sectional view.

INVENTION DESCRIPTION The invention involves selecting samples of shale cuttings from the drilling fluid during drilling operations such that the samples are representative of shale being drilled V 3,494,188 Patented Feb. 10, .1.970

at successive depths, measuring at least one characteristic of shale cuttings within each of the samples, and employing the characteristic as an indication of sub-surface conditions, such as pressureslikely to be encountered. The characteristic so determined may indicate that casing should be set to a lower depth, that mud weight should be raised, etc. y

Bulk density, graindensity, salinity of interstitial water, and the like may serve as indications of the sub-surface conditions. Bulk density, however, is the characteristic preferred for use inthe prediction of sub-surface conditions.

Bulk density as a characteristic of shale cuttings has been employed in well logging with good results. Increases in pressures about to be encountered were accurately predicted, and mud weights were increased and casing was set to lower depths as required in response to the predictions.

Although the theory behind bulk density of shale cuttings as an indication of sub-surface pressures is not fully understood, itis believed to result from water or other liud assuming an abnormal part of the weight applied on a shale stratum from materials overlying the stratum. The abnormal load is believed to be the result of a tendency toward compaction of the shale in the strat-um, compaction being basically a reduction in pore space within the shale. For a reduction in pore space to occur, water or other fluid in the pore space must be squeezed out during the compaction processl to adjacent available spaces. The amount of abnormal pressure depends uponthe ease with which the water or other fluids can escape from the pores in the stratum under compaction to adjacent available spaces,

In thick shale sections of low permeability, for example, since the fluids in the pore spaces can escape only slowly and with difliculty, the fluids are under abnormally high pressures from the weight of overlying strata.

Consequently, if one encounters a shale stratum which has an unusually low bulk density (a low degree of compaction), one lmay predict that the low bulk density is am indication of abnormally high pressures in the stratum. Whether the theory is truc or not, bulk density characteristics of shale can be employed to indicate unusually high pressures.

The bulk density of shale will normally increase with depth, and one can estimate the normal rate of increase in a particular Well or area upon preparation of a log of bulk density against depth in the well or area. A sudden, unusual, or .marked decrease in bulk d'ensity from the normal trend indicates that abnormally high pressures are likely to be encountered. In order to maintain a current log and predict unusual pressures in advance, logging, including the determination of the sample characteristic, should be performed at the well site; and the log kept current as drilling proceeds.

Bulk density for well logging purposes may be obtained by the following process. Shale cuttings from the drilling operation may be obtained'from the drilling iiuid in any suitable manner, such as in suitable screens or traps, for example, a conventional shale shaker. The cuttings are washed in a suitable washing material suchas water (preferably kerosene if oil-base mud), and then screened to obtain the smaller particles present in the drilling fluid. One should select the smaller particles available in order to ensure that the sample particles originated from the actual cutting operation by the drilling bit. Larger particles, since they are frequently the result of sloughing from uphole, lead to spurious results in the log of bulk density against depth at which the sample was cut. There is of course a small time lag between the time of actual cutting and the time of sampling because of the travel time of the drilling fluid uphole, but

this time lag can of course be estimated from the flow rate of the drilling uid if considered significant.

The selected sample after washing is then preferably dried by first placing the sample on a suitable absorbent material, such as a paper toweling, and then placing the sample on a screen in a current of warm air until the uid sheen from the washing material is olf the sampleI and a dull shade is recognized. A conventional hair dryer may be employed as a supply of warm air. At this point, the washing material has evaporated from the apparent surfaces of the cuttings. Too much drying causes fluids from within the pore spaces of the cuttings to evaporate, and the test results will be spurious in that the determination should indicate the bulk density of the shale as it exists in the formation where its filled with oil, water, or other fluids. Too little drying is of course undesirable since inclusion of washing material as a part of the sample in subsequent steps leads to spurious results.

After the above preparatory steps, the desired characteristic of the shale sample can be obtained. In the case of bulk density, the method preferred is to obtain the weight of the sample, measure the bulk volume of the sample, and then calculate the bulk density of the cuttings within the sample.

Apparatus for measuring bulk density of shale cuttings is illustrated `in the drawings. As there shown, a pressure cylinder has a conduit 12 extending vertically from the cylinder 10, the conduit 12 having a sarnple cup holder 14 disposed at the top end thereof such that the holder 14 communicates through the conduit 12 with the cylinder 10. The conduit 12 includes a reservoir 16 for purposes described hereinafter.

The holder 14 has a cap 18 thereon in fluid-tight engagement, the cap 18 including handle 20 for convenience and for permitting accurate setting of the cap 18 in the same location after removal and replacement for maintenance of a constant volume Within the pressure system. The cap 18 also includes a suitable valve 22, such as a needle valve, for opening to the atmosphere, thereby permitting existence of barometric conditions within the holder 14 upon beginning analysis.

A sample cup 24, preferably made of plastic or other material nonwettable by mercury, is disposed in the holder 14. The cup 24 contains shale cuttings 26. The cup 24 is of known weight to permit weighing the shale cuttings 26 on suitable weighing apparatus (not shown), such as conventional weighing scales.

A piston 28 extends into the cylinder 10 through a packing gland 30. A screw 44 engages the piston 28 and controls the movement of the piston 28 within the cylinder 10.

The piston 28 includes at its outermost end a radially expanded portion 32 fitting slidably on guide rods 34 and 36 for stability purposes. Guide rod 34 is mounted between bracket 38 and support 42 while guide rod 36 is mounted between bracket 40 and support 42. A scale 56, which may be conveniently graduated in cubic centimeters, may also be mounted between brackets 38 and support 42. An indicator 58 for the scale 56 is mounted on the portion 32 of the piston 28.

The screw 44 is mounted by means of a thrust bearing 46 to support 42. Gear 48, which is fixedly mounted on screw 44, is engaged with a gear 50 on shaft 54. Shaft 54 also has mounted thereon a handwheel 52 to permit manual operation of the apparatus. A circular scale 60, which may be conveniently graduated in 0.01 cubic centimeters, is iixedly mounted near the outer end of screw 44, and an indicator 62 is provided for scale 60. The indicator 62 may be suitably supported by the shaft 54 or any other convenient supporting means. A pressure gage 64 is mounted on the cylinder 10 in communication with the interior of the cylinder 10.

The cylinder 10 or other pressure container, the piston 28, the conduit 12, and the sample cup holder 14 provide a fluid-tight system. Disposed in the cylinder 10 and part of the conduit 12 is an incompressible liquid, preferably mercury. The cylinder 10 and conduit 12 should contain suicient mercury such that when the piston is in fully retracted position a part of the conduit 12 is filled with mercury, as indicated in the drawing. The conduit 12 should be of sufficient cross-sectional size and length that when the piston 28 is in fully inserted position the maximum level of the mercury will be below the.

top of the cup 24, as indicated by the dots between the cup 24 and the holder 14. In this regard, a reservoir 16 included in the conduit 12 may be employed advantageously.

The bulk density of a sample can be obtained by placing the sample in the cup 24, and obtaining the weight of the sample from the weight of the sample and the cup minus the known weight of the cup. The bulk volume of the sample is determined by obtaining a reference reading on the scales 56 and 60 at a selected pressure on the pressure gage 64 with an empty sample cup 24 in place in the holder 14, then obtaining a reading at the same pressure with the sam-ple in the cup 24, and then determining the bulk volume by the difference in readings. Bulk density of course is easily calculated from the weight of the sample divided by the bulk volume.

Preferably, the determination procedure also includes placing a known volume of steel balls or other suitable material in the sample cup 24 empty of sample, obtaining a reading in cubic centimeters on` the scales A56 and 60 at the selected pressure, and then determining a correction factor from this reading and the reference reading obtained without sample in the sample cup 24. Th-e correction factor is desirable since it helps correct for variations in atmospheric conditions. Of course, if a controlled atmosphere were maintained in the sample cup holder 14, a correction factor would not be required, but in operation at a rig site, provision of a controlled atmosphere and the attendant apparatus is ordinarily too inconvenient and expensive to be desirable.

The above apparatus is convenient to operate, and

. gives accurate results. It also withstands the rough usage andv severe vibrations which ordinarily exist under field conditions. In this regard, the size of the conduit 12 and the existence of the sample cup 24 is particularly important in that mercury is easily contaminated by shale cuttings and the presence of shale in the mercury leads to spurious results in measurement of bulk volume. Consequently, the level of mercury should be maintained such that mercury does not contact the shale cuttings during the test. On the other hand, one should avoid trapping air or gas in the pressure cylinder, since entrapped air or gas will cause inaccurate results.

What is claimed is:

1. A method of well logging during drilling and predicting mud .weight requirements when drilling a well through high pressure shale, which method comprises successively obtaining shale cuttings from the drilling fluid during the drilling operation, successively measuring bulk density of shale cuttings successively obtained, logging the bulk density measurements to make a log of the normal increase in bulk density with depth to establish a normal trend of the normal increase in bulk density, and measuring the difference in bulk density between cuttings taken from the high pressure shale and the bulk density predicted by said normal trend to predict the mud weight required.

2. The method defined in claim 1 wherein the cuttings obtained from the drilling fluid are collected from the smaller particles available to ensure that the cuttings represent the material being drilled by the drilling bit.

3. The method defined in claim 1 wherein the bulk density is measured by measuring the weight of cuttings, measuring the bulk volume of those cuttings, and measuring the bulk density from the weight and bulk volume of the cuttings.

4. The method defined in claim 1 wherein measuring the bulk density of the cuttings includes the steps of wash- .f ing cuttings with a washing material, then drying washed cuttings suiciently to remove the washing materialfrom the apparent surfaces thereof, then measuring the weight and the bulk volume ofthe cuttings, and measuring ,bulk density from the weight and bulk volume of the cuttings.

5. The method defined lin claim 4 `wherein the bulk volume of the cuttings is measured by placing the cuttings in a gaseous atmosphere in a pressure chamber, pressurizing the chamber to a selected pressure by moving liquid toward the chamber, and measuring the bullgvolume of the cuttings from the amount of liquid moved to elect the selected pressure as compared with a reference point determined at the selected pressure without cuttings in the chamber. l

6. A method of well logging during drilling, which method comprises successively collecting samples of shale cuttings from the drilling fluid during drilling operations such that the samples are representative olir the shale being drilled at successive depths, measuring successively the bulk density of shale cuttings within each samkple, logging the bulk density of shale cuttings within each sample with respect to the depth of which each sample is representative, and indicating sub-surface conditions during drilling from the resulting log.

7. The method defined in claim 6 wherein the cuttings collected from the drilling fluid are collected from the smaller particles available to ensure that the cuttings represent tbe shale being drilled by the drilling bit.

8. The method defined in claim 6 wherein the bulk density is measured by measuring the weight of cuttings, measuring the bulk volume of those cuttings, and measuring the bulk density from the weight and bulk volume of the cuttings.

9. The method defined in claim 6 wherein measuring the bulk density of the cuttings includes the steps of washing cuttings with a washing material, then drying washed cuttings sufficiently to remove the washing material from the apparent surfaces thereof, then measuring the weight and the bulk volume of the cuttings, and meas-y uring bulk density from the weight and bulk volume of the cuttings. if

10. The method defined in claim 9 wherein the bulk volume of the cuttings is measured by placing the cuttings in a gaseous atmosphere in a pressure chamber, pressurizing the chamber to a selected pressure by moving liquid toward the chamber, and measuring the bulk volume of the cuttings from the amount of liquid moved to effect the selected pressure as compared with a reference point determined at the selected pressure' without cuttings in the chamber.

11. A method of well logging during drilling, which method comprises successively obtaining samples of shale cuttings from the drilling iiuid during drilling such that the samples are representative of the shale being drilled at successive depths, measuring successively at least one characteristic of shale cuttings within each of the samples, logging the characteristic of shale cuttings within each of the samples with respect to the depth of which each sample is representative, and indicating subsurface conditions during drilling from the resulting log.

12. The method defined in claim 11 wherein the cuttings obtained from the drilling tiuid are collected from the smaller particles available to ensure that each sample of cuttings represents the shale then ibeing drilled by the drilling bit.

13. A method of well logging during drilling, which method comprises obtaining shale cuttings from the drilling uid during drilling such that the cuttings are representative of the shale then being drilled, including collecting the cuttings from the smaller particles available in the drilling fluid; washing the shale cuttings with a washing material and then drying the washed cuttings su'ciently to remove the washing material. from the apparent surfaces thereof; weighing a [selected] sample of the shale cuttings; placing the sample in a gaseous atmosphere in a pressure chamber; pressurizing the chamber to a selected pressure by moving liquid toward the chamber; measuring the bulk volume of the sample from the amount of liquid moved to effect the selected pressure as compared with a reference point determined at the selected pressure without the sample in the chamber; vmeasuring the bulk density of the sample from the weight and bulk volume of the sample; and indicating sub-surface conditions during drilling from the bulk density.`

14. The method defined in claim 13 wherein said Washing material is water and said liquid is mercury.

15. Apparatus for measuring bulk density of shale cuttings obtained from the drilling fluid during drilling operations, said apparatus comprising a pressure ycontainer; a movable piston extending into said pressure container; a conduit extending from said container to a position above said container; a sample cup holder disposed above said container and communicating with said pressure container through said conduit; said sample cupholder including a cap to permit location of a sample cup therein; said pressure container, piston, conduit, and sample cup holder providing a tluid-tight system; a sample cup disposed in said holder, said cup having a known weight to permit weighing of a sample disposed therein,

and 'being made of material nonwettable by mercury; mercury filling said container and part of said conduit when said piston is in fully retracted position with respect to said container; said conduit being of sufficient cross-sectional size and length that when said pistonis in fully inserted position the mercury level in the system is below the top of the sample cup; means for measuring pressure within the system; and means for measuring the quantity of mercury moved by movement of said piston.

16. Apparatus adapted for measuring bulk density of shale cuttings obtained from the drilling uid during well drilling operations, said apparatus comprising a pressure container; a movable piston extending into said pressure container; a conduit extending from said container;

`upper portion of said sample cup holder, said cup lhaving a known weight to permit weighing of a sample disposed therein, and being made of material nonwettable by mer.- cury; mercury filling said container, said conduit, and said sample cup holder to bring the mercury level in the system Ibelow the top of said sample cup when said piston is in an inserted position and to lower said mercury level in said liuid-tight system when said piston is in a retracted position with respect to said container; means for measuring pressure within said fluid-tight system; and means for measuring the quantity of mercury moved by movement of lsaid piston between its said retracted and inserted positions,

17. Apparatus adapted for measuring bulk density of shale cuttings obtained from the drilling fluid during welldrilling operations and comprising a first pressure container; a piston member extending into said first pressure container and adapted Vfor movement relative thereto between retracted and inserted positions; a second pressure container having an upper portion extending above said first pressure container, said second pressure container including an access opening therein to permit location of a sample cup in said upper portion thereof and a removable coveradapted for sealingly closing said access tion of said second pressure container, said sample cup having a known weight to permit weighing of a sample disposed therein, and being made of material nonwettable by mercury; mercury filling said pressure container, said conduit means, and said second pressure container to a level below the top of said sample cup when said piston member is in its said inserted position; means for measuring pressure Within said fluid-tight system; and means for measuring the quantity of mercury moved by movement of said piston between its said retracted and inserted positions.

18, Apparatus adapted for measuring Ihulk density of shale cuttings obtained from the drilling 4iiuid during well-drilling operations and comprising pressure-containing means defining an enclosed upper air chamber and a liquid chamber below said enclosed air chamber in which a non-compressible liquid can be progressively elevated for correspondingly compressing air trapped in said enclosed air space in relation to the depth of such a liquid in said liquid chamber; an open sample receiver disposed in said enclosed air chamber and adapted to contain such shale cuttings; liquid-displacement means operatively coupled to said pressure-containing means and including a piston chamber uidly coupled to said liquid chamber, a piston member movably received in said piston chamber and adapted for movement therein between a retracted position and an inserted position, and a non-conipressible liquid filling said piston chamber and said liquid chamber to a depth below said sample receiver when said piston member is in its said inserted position; pressuremeasuring means for measuring the pressure developed in said enclosed air chamber; and volume-measuring means for measuring the quantity of said non-compress# ible liquid displaced by said piston mem-ber upon movement thereof between its said retracted position and its said inserted position.

References Cited UNITED STATES PATENTS Horner 73-149 X 

